Phosphorus-Carbon Bond Formation Using Metal-Coordinated Phosphenium Ions and Phosphine Triflates PDF Download

Author: Arumugam Jayaraman
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Languages : en
Pages : 0

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Book Description
Chloride abstraction reactions have been investigated towards the formation of metal-coordinated phosphenium ions ([M]-PR2+) from metal-coordinated chlorophosphines. The electron-poor metal fragment W(CO)5 was used on all studied chlorophosphine complexes. Chlorophosphines [W(CO)5{P(Cl)C(Ph)C(Ph)}] (4), [W(CO)5{PPh2Cl}] (32), [W(CO)5{PPhCl2}] (60), [W(CO)5{PCl3}] (100) and [W(CO)5{P(NEt2)Cl2}] (101) were synthesized using existing or new methods. Chloride abstraction from 4 with excess AlCl3 leads to the phosphirenyl cation complex [W(CO)5{PC(Ph)C(Ph)}][AlCl4] (8). Compound 8 undergoes electrophilic aromatic substitution with ferrocene to form a ferrocenylphosphirene complex. Chloride abstraction from 4 with AgOSO2CF3 leads to a phosphirenyl triflate complex 12, which reacts with ferrocene in the same fashion as 8 but also reacts cleanly with a wider range of substrates, including phenylacetylene, allyltrimethylsilane, activated to moderately-activated benzene derivatives, and heteroaromatic compounds and formed P-C bond at the expected positions. Most of the substitution reactions are rapid at ambient temperature, regioselective, and the product yields were good to excellent. Chloride abstraction from 32 using AgOSO2CF3 led to the formation of diphenyl phosphine triflate complex 34. However, with AlCl3 an equilibrium mixture of the isocarbonyl complex [W(CO)4{COAlCl3}{PPh2Cl}] and phosphenium complex [W(CO)5{PPh2}][AlCl4] (33a) were formed. With GaCl3, only the phosphenium complex 33b was formed. Compounds 34 and 33a react in a similar fashion to phosphirenyl triflate complex 12 with various aromatic substrates, and showed an increased reactivity than 12. In addition to aromatic and heteroaromatic substrates, 34 and 33a react with alkenes, alkynes and ketones. With alkenes and alkynes, P-C bonded products were formed, but with ketones P-O bond formation is predominant. Reaction of 60 with AlCl3 resulted in the disappearance of 60 in the 31P{1H} NMR spectrum, but no detectable signal for the phosphenium ion complex. Addition of activated aromatic substrates to this solution leads to disubstitution. The AgOSO2CF3 selectively converts compound 60 to the mono-substituted phosphine-triflate complex [W(CO)5{PPhCl(OSO2CF3)}] (62), which reacts with various aromatic substrates, including less activated ones, to afford a variety of phenyl aryl chloro complexes. Most of the phenyl aryl chloro phosphine complexes were subsequently converted to corresponding triflates and reacted with ferrocene to show the possibility of sequential substitution of two chloride substituents with two different carbon-based substituents. Complex 100 was used to test the possibility of substituting all three Cl substituents of P with three different carbon-based-substituents, but chloride abstraction was not possible from this complex. Therefore, compound 101 was examined as an alternative. In the presence of AlCl3, it reacts successively with allyl trimethylsilane, indole and ferrocene to form the 3-indolyl allyl ferrocenyl phosphine complex in a single pot. Phosphine products can be removed via oxidation of W with I2, followed by displacement with bipyridine, or by photolysis of the THF solution containing phosphine complex and bis(diphenylphosphino)ethane. Density Functional Theory calculations were also performed to assess the relative electrophilicities of the studied phosphenium ion complexes.